Research has established that healing of wounds such as bums, skin ulcers, pressure sores and traumatic injuries is facilitated when the wound bed is kept moist and clean. Moist wound dressings are particularly useful for this purpose and have become an accepted therapy for treating wounds. In this context, moist means that the dressing keeps the wound moist, and not necessarily that the dressing is moist when applied to the wound. It is postulated that these dressings promote optimum physiological conditions for healing in the wound by maintaining or promoting tissue hydration. When applied to dry wounds, the dressings rehydrate dessicated tissue, either by preventing loss of water vapor from the site or by directly transferring moisture to the tissue. When applied to exudating wounds, the dressings absorb the exudate and promote hydration of tissue. Autolytic debridement of necrotic tissue and/or formation of new tissue occurs more readily under these conditions. In addition, a variety of growth factors that promote wound healing are present in the exudates from the wounds (see Howell, J. M., Current and Future Trends in Wound Healing, Emerg. Med. Clin. North Amer., 10, 655-663 (1992)), and it is believed that moist wound dressings that can absorb fluids from the exudate promote healing by minimizing loss of these growth factors from the wound bed.
Several types of moist wound dressings are commercially available, including hydrogels, hydrocolloids, semipermeable adhesive films, perforated films, alginates, polysacside beads, an d poly urethane foams. These types are distinguished by the physical form of the dressing, by its mechanism of action, and by its chemical composition.
Hydrogel dressings are composed of insoluble polymers having hydrophilic sites that interact with aqueous solutions, and can absorb and retain a significant volume of fluid. Use of these dressings is growing at a double digit rate, driven by an increasing elderly population afflicted with chronic wounds such as skin ulcers, due to diabetes, or pressure sores, resulting from being bedridden.
Hydrogel dressings have additionally been used as carriers for the delivery of therapeutic agents to a wound site, usually for the treatment of infection. For example, Intrasite gel, an amorphous hydrogel wound treatment manufactured by Smith & Nephew, is approved in the United Kingdom as a carrier for metronidazole for the treatment of fungating and other malodorous wounds. Generally, a medicament or drug used as the therapeutic agent is incorporated in the hydrogel during manufacture of the dressing, or, for film-type dressings, taken up into the polymer by swelling a dry film with an aqueous solution of the therapeutic agent. After the dressing is applied to the wound, the therapeutic agent diffuses into the tissue. It is expected that such therapies that combine treatment of wounds with moist wound dressing with delivery of a drug, especially an antibiotic, would provide a significant benefit to patients. Unfortunately, use of hydrogels as carriers has been severely limited by the composition and resulting physical properties of available products. Many of the commercial moist wound dressings are composed of a crosslinked ethylene oxide polymer. These dressings are typically manufactured by irradiating an aqueous solution of a functionalized polyethylene oxide with ionizing radiation, resulting in a sheet of insoluble gel swollen with water. Any drug to be incorporated prior to the crosslinking step must be stable to this high-energy radiation. Alternately, it is possible to dehydrate the gel following crosslinking and rehydrate with an aqueous solution of the drug. However, dressings composed of polyethylene oxide frequently develop unacceptable cosmetic defects when dehydrated and rehydrated.
"Therapeutic agent," as used herein, includes drugs and medicaments for treatment of pathological conditions and for prophylactic use. Included within the definition are antibacterial agents, anesthetics, cell adhesion peptides, such as RGD peptides, growth factors, spermicides, antiviral agents, antifungal agents, antiparasitic agents, anti-inflammatory agents, antihistamines, analgesics, antineoplastic agents, hormones, kerolytic agents, tranquilizers, vitamins, base-pair nucleotides and cytokines.
In addition, metals having oligodynamic properties are herein defined as therapeutic agents. The antimicrobial effect of certain metals or metal ions at very low concentrations is termed oligodynamic action. Metals having oligodynamic properties include silver, gold, platinum, palladium, mercury, copper, tin, antimony, bismuth, zinc, aluminum and magnesium. Ions are the most active form of these metals. The antibacterial effect of an oligodynamic metal may be increased by electrically injecting the metal ions into solution. This process has been termed oligodynamic iontophoresis. lontophoretic structures for medical devices have been described in U.S. Pat. No. 5,322,520 and related patents. The patents teach imbedding two dissimilar metals in polymers filled with carbon or other conductive fillers. Ions are forced into the conductive fluid environment using the minute electric currents generated by the galvanic potential difference between the two metals. The dissimilar metals act as electrodes, with a voltage potential therebetween, whereby electrons migrate through the polymer, generating an electric current. Metal ions are driven into the conductive matrix by this very small electric current.
The antimicrobial action of silver ion is particularly well known. For example, silver compounds, such as silver sulfadiazine for treatment of burns, are used routinely in antibacterial salves, and has also been used to coat gauze for burn dressings. Medical devices, such as catheters, coated with a silver-impregnated collagen or polymer are also known. However, there is a need for improved methods of delivering therapeutic agents, including oligodynamic metals, to a site susceptible to infection.
U.S. Pat. No. 5,840,387 to Berlowitz-Tarrant et al. discloses use of a sulfonated copolymer of styrene for delivery of non-silver therapeutic agents. The patent does not disclose a silver-containing polymer or a moist wound dressing comprising a styrene sulfonate polymer.